1. Field of the Invention
The present invention generally relates to an image forming apparatus. More particularly, a certain aspect of the present invention relates to an image forming apparatus including a recording head for jetting liquid droplets.
2. Description of the Related Art
A liquid-jet image forming apparatus such as an inkjet recording apparatus uses one or more recording heads for jetting ink droplets to form an image. A liquid-jet image forming apparatus is used, for example, for a printer, a facsimile machine, a copier, a plotter, and a multifunction copier having functions of them. Such a liquid-jet image forming apparatus jets liquid droplets from its recording heads onto paper being conveyed (“paper” in the present application is not limited to a sheet of paper but also refers to any medium, such as an OHP sheet, to which ink droplets or other liquid droplets can adhere, and may also be called a recording medium, recording paper, recording sheet, etc.), and thereby forms (records or prints) an image on the paper. There are roughly two types of liquid-jet image forming apparatuses: a serial-type image forming apparatus including a recording head that jets liquid droplets while moving in the main-scanning direction to form an image; and a line-type image forming apparatus including a line-type recording head that remains stationary while jetting liquid droplets to form an image.
In the present application, a liquid-jet image forming apparatus refers to an apparatus that forms an image by jetting a liquid onto a recording medium made of paper, thread, fabric, textile, leather, metal, plastic, glass, wood, ceramic, etc. Also, “image forming” indicates not only a process of forming a meaningful image such as a character or a drawing on a recording medium, but also a process of forming a meaningless image such as a pattern on a recording medium (or just jetting liquid droplets onto a recording medium). Further, in the present application, “ink” refers not only to an ink (colored liquid) in a general sense, but also to any liquid usable for image forming such as a recording liquid, a fixer solution, a DNA specimen, a resist, or a pattern material.
Generally, in a liquid-jet image forming apparatus (hereafter, may also be simply called an inkjet recording apparatus), a cleaning process is performed as needed to remove bubbles and dried ink from nozzles of a recording head and thereby to prevent nozzle clogging. In the cleaning process, a nozzle surface is enclosed (or covered) by a cap, and a space formed by the nozzle surface and the cap is depressurized by a suction pump (suction unit) connected to the cap to forcibly evacuate bubbles and dried ink from nozzles.
After the cleaning process is completed, the suction pump is stopped, the cap is kept over the nozzle surface for a predetermined period of time so that the cap is filled with ink and the negative pressure in the cap is reduced, and then the cap is moved away from the nozzle surface. This makes it possible to prevent a large pressure change from occurring when the cap is moved away.
For example, patent document 1 discloses an inkjet recording apparatus including a pressure pump disposed in a channel connected to an ink cartridge used as a liquid container. The pressure pump forces air into the ink cartridge and thereby pressurizes ink in the ink cartridge. After a head is caused by a suction pump to jet ink into a space in a cap, the pressure pump is driven to apply positive pressure to the ink in the head in the ink-jetting direction and then the cap is moved away from the nozzle surface.
[Patent document 1] Japanese Patent Application Publication No. 2005-225163
Meanwhile, even if a cap is moved away from a nozzle surface after ink is suctioned and the negative pressure in the cap is decreased as described above, ink and bubbles adhering to the nozzle surface may be drawn into nozzle channels because of low residual pressure in the head caused by suction or negative pressure caused by meniscus formation, and may cause nozzle clogging.
Applying positive pressure to ink in a head before moving a cap away from a nozzle surface as in patent document 1 makes it possible to prevent ink and bubbles from being drawn into nozzle channels. However, with the configuration of patent document 1, since an ink cartridge for supplying ink to a recording head is disposed between a pressure pump and the recording head, pressurized residual air remains in the ink cartridge even after the pressure pump is stopped and therefore ink drips off the nozzles of the recording head after the cap is moved away. The dripping ink adheres to the nozzle surface and makes it difficult to clean the nozzle surface even with a wiping part.